Alignment system for multiple color ink jet printheads and associated printhead with built-in optical position detector

ABSTRACT

The individual monochromatic printheads mounted simultaneously on the scanning carriage of an ink jet printer are provided with an opto-electronic position detector of the four-quadrant type, which is integrated directly in the chip of each head by means of the same MOS technology used for the circuits for selection and driving of the emission resistors; an illumination device fixed onto the printer focuses a light spot on the position detector which thus provides the electronic control unit of the printer with a measurement of both the vertical and horizontal misalignment of each head, a measurement which is subsequently used in order to compensate automatically for the misalignment thereof.

TECHNICAL FIELD

The system refers to an alignment system for multiple colour ink jetprintheads in a dot matrix printer comprising a fixed structure, ascanning carriage for supporting the printheads, which is movable withrespect to the fixed structure in a first horizontal direction, and anelectronic control unit comprising a function for timing the emission ofink droplets by the heads.

BACKGROUND ART

Colour ink jet printers both of the “thermal” type and of the“piezoelectric” type are widely known, said printers being provided witha plurality of monochromatic heads (typically three or four) which arephysically and mechanically identical to one another, but containdifferent inks (typically corresponding to the primary colours blue,yellow and magenta, and in some cases black); each head has a largenumber of nozzles for emitting ink droplets (for example, three hundred,but the current trend of technology is towards even larger numbers)which are arranged at uniform intervals in one or more parallel rows,with a corresponding number of emission elements for producing the inkdroplets which are selectively ejected through the nozzles.

As is known in the latest technology, ink jet printheads of the“thermal” type comprise a semiconductor (usually silicon) substrate orchip having, formed on it, using known technologies, both the componentswhich more specifically relate to ink jet technology (the emissionresistors) as well as the power drivers for driving them and the logicfor selecting the individual emission resistor to be driven; for thefirst group of components normally thin-film technology is used, whilefor the second group LDMOS (lateral double diffused MOS) technology andfor the third group CMOS technology are used, these latter technologiesbeing used in the simplified version with the least number of processsteps and masks, so as to satisfy only the functional requirements ofthe bubble ink jet heads.

The relative positioning precision of the nozzles with respect to oneanother on an individual head is very high, since the nozzle-carryingplate is made as one piece and the active part of the head is formed ona single silicon chip, all of which using micro photolithographictechniques which ensure a high degree of mechanical precision. The samecannot be said about the positioning precision with which the chip isassembled on the container body of the head, the latter being in turnmounted on the scanning carriage of the printer, such that finalalignment of the nozzles on the various monochromatic heads (necessaryfor obtaining a good print quality, in particular in high definitionmode, as is well known to those skilled in the art) may be obtained onlyby means of additional operations involving operational alignment of theheads, to be performed in a more or less automatic manner, directly onthe printer, with consequent difficulties of a practical andcost-related nature.

DISCLOSURE OF THE INVENTION

The object of the present invention is that of defining a system forobtaining operational alignment, both in the horizontal direction(scanning direction) and vertical direction (interlinear feeddirection), of the printheads of a colour ink jet printer havingmultiple monochromatic heads, with the necessary precision forhigh-quality high-definition colour printing.

The object of the present invention is obtained by the alignment systemfor multiple colour ink jet printheads and associated printhead withbuilt-in optical position detector, characterized by the steps ofproviding printheads comprising a built-in opto-electronic positiondetector, for detecting a first misalignment of the heads in the firsthorizontal direction and a second misalignment of the heads in a secondvertical direction, substantially perpendicular with respect to thefirst horizontal direction, the heads also comprising a plurality ofnozzles arranged at uniform intervals in at least one row parallel tothe second vertical direction; providing an illumination device integralwith the fixed structure for producing a light spot focused on theopto-electronic detector; detecting the first misalignment and thesecond misalignment by means of the combined use of the built-inopto-electronic position detector and the illumination device;compensating for the first misalignment of the heads in the firsthorizontal direction, by means of a variation of the timing of theemission of ink droplets, and compensating for the second misalignmentof the heads in the second vertical direction by means of a verticaldisplacement of the heads.

The system according to the invention is based on the availability ofprintheads comprising at least one opto-electronic device which forms anoptical position detector of the four-quadrant type, integrated on thechip itself of the head, i.e. manufactured during the course of the sameproduction process, using standardized technologies for the productionof integrated semiconductor circuits, with which, based on the commonsilicon substrate, the other components necessary for operation of thehead itself, such as the emission resistors, the selection circuits, thedrivers and the connection conductors, are made.

In this way the integrated opto-electronic device forms an opticalposition detector which is aligned with the nozzles withphotolithographic precision and by means of which it is possible todetect automatically both the horizontal position and the verticalposition of each individual monochromatic head mounted on the scanningcarriage; the system according to the invention uses the positiondetection operations thus performed in order to make the appropriatecorrections needed to compensate for the geometric alignment errorsdetected, by means of the electronic control unit of the printer.

The horizontal alignment errors are corrected by suitably delaying oradvancing the timing of emission of the ink droplets by the variousmonochromatic printheads with respect to the instant determined on thebasis of the theoretical position of the head itself and the speed ofdisplacement of the carriage; the vertical alignments errors arecorrected, on the other hand, by suitably modifying the physicalposition of the heads (for example, by vertically displacing the head bya suitable amount by means of a piezoelectric micromotor) or else(assuming a maximum misalignment one half pitch) by dispensing with useof the nozzles of each head located outside a common alignment band.

Another object of the present invention is that of defining a system foradjusting both the horizontal and vertical mechanical alignment of theseats supporting the printheads of an ink jet colour printer havingmultiple heads, with the precision necessary for high-definition colourprinting, during testing of the printer itself or during a maintenanceoperation, using a sample printhead.

A further object of the invention is that of defining a system foradjusting both the horizontal and vertical mechanical alignment of thechip+flat cable assembly subgroup with respect to the container of aprinthead for a colour ink jet printer having multiple heads, duringconstruction of the head itself.

Another object of the invention is that of defining a system foradjusting both the horizontal and vertical mechanical alignment,relative to one other, of two or more chips which make up a multipleprinthead (able to print in a single pass a strip with a heighttypically greater than one inch) for particular applications (forexample, postal franking).

Another object of the invention is that of defining a printhead providedwith an optical position detector integrated on the same semi-conductorchip and using the same technologies used to manufacture the emissionelements and the logic and power circuitry.

The abovementioned objects are achieved by means of a system foroperational alignment of the monochromatic printheads of a colour inkjet printer provided with multiple heads, and by means of a printheadwith a built-in optical position detector, which are characterized asdefined in the main claims.

These and other objects, characteristic features and advantages of theinvention will emerge clearly from the following description of apreferred embodiment thereof, provided by way of a non-limiting example,with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the simplified electrical diagram of the opto-electronicdevice integrated on the chip of the printhead according to theinvention in order to detect alignment thereof;

FIG. 2 shows in schematic form a cross-sectional view of theopto-electronic device according to FIG. 1;

FIG. 3a shows in schematic form a plan view of the sensitive area of theopto-electronic device according to FIG. 1;

FIG. 3b shows in schematic form a plan view of the sensitive area of theopto-electronic device according to FIG. 1, with the addition of acalibration phototransistor;

FIGS. 4a-4 g show in schematic form seven different positions of thephotosensitive area of the opto-electronic device according to FIG. 1with respect to the light spot of the light beam focused by anillumination device;

FIG. 5a shows in diagrammatic form the pattern of the voltages V_(A) andV_(B) obtained from the emitter photocurrents of the twophototransistors T_(A) and T_(B) in the seven positions of thephotosensitive area of the opto-electronic device of FIG. 1 with respectto the light spot of the light ray focused by an illumination deviceshown in FIGS. 4a-4 g; and

FIG. 5b shows in diagrammatic form the pattern of the difference(V_(A)−V_(B)) of the voltages V_(A) and V_(B) shown in FIG. 5a.

BEST MODE FOR CARRYING OUT THE INVENTION

The alignment system according to the invention, which is intended toobtain both the horizontal and vertical operational alignment of theprintheads of a colour ink jet printer having multiple monochromaticheads mounted on the scanning carriage with the necessary precision forhigh-definition colour printing, requires the availability, in additionto that which is normally present in a similar printer according to theknown art, of:

a) printheads provided with a four-quadrant optical position detectorwhich is built-in, i.e. manufactured during the course of the sameproduction process, using standardized technologies for the productionof integrated semiconductor circuits, with which, based on the commonsilicon substrate, the other components necessary for operation of thehead itself, such as the emission resistors, selection circuits, driversand connection conductors, are made;

b) an illumination device fixed onto the printer;

c) a device for performing vertical displacement of the printheads;

d) an electronic control unit able to process the signals produced bythe optical position detector and to modulate timing of the emission ofthe ink droplets according to the signals thus processed;

in which the characteristic features which are of importance for thepurposes of the present invention will be described in more detailbelow.

Printhead—The printhead according to the invention is an ink jet head ofthe multiple-nozzle thermal type, with selection and driver circuitsmanufactured using CMOS and LDMOS technology and droplet generatingcomponents manufactured using thin-film technology and integrated on asingle support (semiconductor substrate or chip), of the type known inthe art, which also comprises an opto-electronic device 10, theelectrical diagram of which is shown in FIG. 1 and which forms aposition detector of the four-quadrant type which is also integrated onthe same support and manufactured using the same semiconductorintegrated-circuit technologies mentioned above.

From the electrical point of view, the opto-electronic device 10consists of four phototransistors T_(A), T_(B), T_(C) and T_(D) whichare identified respectively by the numbers 30, 40, 50 and 60 and whichhave open bases B_(A), B_(B), B_(C) and B_(D) which are identifiedrespectively by the numbers 31, 41, 51 and 61; common collectors C_(A),C_(B), C_(C) and C_(D) which are identified respectively by the numbers33, 43, 53 and 63 and connected electrically to one another at a commonnode 34 from where they receive a power supply of +35V; and independentemitters E_(A), E_(B), E_(C) and E_(D) identified respectively by thenumbers 32, 42, 52 and 62.

The photocurrents i_(A), i_(B), i_(C) and i_(D), generated by thephototransistors T_(A) 30, T_(B) 40, T_(C) 50 and T_(D) 60 on thecorresponding emitters E_(A) 32, E_(B) 42, E_(C) 52 and E_(D) 62 whenthe associated bases are suitably illuminated, are individuallycollected by four separate amplifiers known per se, not shown in thefigures, which are preferably integrated on the same chip as the headand which produce by way of response four output voltage signals V_(A),V_(B), V_(C) and V_(D) which are proportional to the individualphotocurrents i_(A), i_(B), i_(C) and i_(D) and of a suitable level (forexample, variable between 0 and +5 V). The voltage signals V_(A), V_(B),V_(C) and V_(D) output by the amplifiers are input to standardizedelectronic circuits (for example, differential amplifiers andcomparators) which are in turn also preferably integrated on the samechip as the head, but which functionally form part of the electroniccontrol unit of the printer, for subsequent processing thereof

The physical structure of the opto-electronic device 10 is shownschematically in FIG. 2 by means of a cross-sectional view which shows asingle pair of phototransistors (for example T_(A) 30 and T_(D) 60). Thepair of vertical phototransistors T_(A) 30 and T_(D) 60 is formed by theN+ zones representing the collectors 33 and 63 connected to a commonoutput 34, by the P-type bodies 31 and 61 representing the open basesand by the N-type layers 32 and 62 representing the emitters. Said pairis delimited by an N-type well 21 which is formed by means of diffusionon a P-type silicon substrate 20 in a zone of the latter not protectedby an SiO₂ field oxide (Locos—local oxidation of the silicon substrate)layer 25; the opto-electronic device 10 is then protected by aprotective passivation layer 26 (BPSG—boron/phosphorus siliceousglassification), except for the areas where the metal layers which formthe output conductors 34 for the collectors, and 36, 66 for theemitters, are deposited.

The other two vertical phototransistors T_(B) 40 and T_(C) 50 of theopto-electronic device 10 of FIG. 1, with common collector and open baseidentical and adjacent to those described above, are integrated on thesubstrate 20 in the direction perpendicular to the plane of FIG. 2 so asto form overall an optical position detector with four quadrants.

The geometrical configuration of the active areas of the verticalphototransistors T_(A) 30, T_(B) 40, T_(C) 50 and T_(D) 60 forming theopto-electronic device 10 is represented by means of the plan view ofFIG. 3a; the four quadrants consist of the photosensitive areas A, B, Cand D corresponding to the open bases B_(A) 31, B_(B) 41, B_(C) 51 andB_(D) 61, and each of the photosensitive areas A, B, C and D has, by wayof example, a rectangular shape, the dimensions of which are preferablycomprised within the following limits:

height 100-500 μm

width 10-200 μm

Moreover, the photosensitive areas A and B, and C and D, respectively,are adjacent to one another along the smaller dimension and, in turn,facing one another along the larger dimension following a verticalstraight line L so as to form overall a single rectangle in which theindividual photosensitive areas are arranged in succession in theclockwise direction in the order A→B→C→D and the straight line Lrepresents the axis of vertical symmetry. By way of example, if theindividual areas A, B, C and D have a dimension of 250×50 μm, saidsingle rectangle has dimensions of 500×100 μm.

According to a variant of the invention, the opto-electronic device 10,in addition to the vertical phototransistors T_(A) 30, T_(B) 40, T_(C)50 and T_(D) 60 also comprises a calibration phototransistor, thefunction of which will be described below and which is made using thesame technology described above, with a photosensitive area E ofrectangular shape which is aligned with the photosensitive areas A, B, Cand D with the larger side (height) parallel to the vertical straightline L, as shown in FIG. 3b, and with a dimension equal to twice theheight of the photosensitive areas A, B, C or D, and with the smallerside of equal width.

By way of example, the values of the doping operations and thethicknesses of the corresponding layers used to produce the device 10,which are typical of CMOS/LDMOS technology, are as follows:

N+ emitter: > 10²⁰ cm⁻³; xj = 0.5 μm, P body: ≈ 1 × 10¹⁷ cm⁻³; xj = 1.5μm, N well: ≈ 1 × 10¹⁶ cm⁻³; xj = 4.5 μm, P substrate: ≈ 1 × 10¹⁵ cm⁻³.

It should be pointed out that the opto-electronic device 10 thusobtained is not optimized compared to solutions aimed at obtaining thebest characteristics (for example, response in terms of frequency,noise, sensitivity, etc.), but it nevertheless has good functionalcharacteristics and is obtained in particular using the same processsteps and the same masks which in any case are necessary for manufactureof an integrated “thermal” ink jet head, i.e. without an increase incosts and difficulties compared to the known heads. Moreover, with theprocess described it is possible to achieve optimum reproducibility ofthe photoelectric characteristics of the vertical phototransistors T_(A)30, T_(B) 40, T_(C) 50 and T_(C) 60, since they depend essentially ondoping of the P body 21 and the P body/N well junction, such thatdispersion of the values of the emitter photocurrent among the variousphototransistors integrated on the same chip is less than ±2%, while thedispersion of the emitter photocurrent among the opto-electronic devices10 integrated on various chips is limited to within ±10% in the casewhere doping of the N and P areas is performed by means of ionimplantation with doping control better than ±5%.

But the main advantage obtained from having the opto-electronic device10 integrated on the chip of the head is the high degree of precisionwith which the device itself is positioned with respect to the nozzlessince the whole assembly is formed on a single silicon substrate, usingmicro-lithophotography techniques which ensure considerable mechanicalprecision, so as to form an optical position detector which is optimizedfor its intended use.

Illumination device. This consists of a fight source, typically aninfrared fight-emitting diode (LED) or a laser diode, which are knownper se, with emission on a wavelength band centred on about 900 nm,typically an illumination device of this type provides a luminousintensity of about 0.25 mW at 800 nm with an electric power consumptionof about 10 mW. Using a known lens system (for example, a “Multielementcollimating lens”, produced by Optima Precision Inc. USA) and a suitablecircular diaphragm, one obtains a circular light point or spot 30 (FIG.4a) with a diameter of 250 μm focused at a distance of 7-8 mm from thesource and having a uniform illumination intensity over the wholecircular area 30 of about 20 mW/cm².

The illumination device (not shown in any of the figures since it doesnot possess any inventive character and can be easily duplicated bythose skilled in the sector art) is attached to a fixed element (forexample the structure) of the printer in a position so as to project thecircular light spot 30 onto optoelectronic devices 10 which areintegrated on the chips of the individual printheads mounted on thescanning carriage of the printer, when the latter, during its travelmovement, is located in a given position, as shown in FIGS. 4a-4 g.

With the abovementioned illumination intensity value indicated above,each of the vertical phototransistors T_(A) 30, T_(B) 40, T_(C) 50 eT_(D) 60 forming the opto-electronic device 10 (assuming, as typicalvalues for an NPN vertical transistor produced with CMOS/LDMOStechnology, a quantic efficiency of 50% in the wavelength range of700-800 nm and a gain β=40-50), generates a maximum emitter photocurrent(proportional to the illuminated photosensitive area, the lightintensity being uniform) of about 200 μA and a dark current of 10-50 nA.

Device for performing vertical displacement. According to a preferredembodiment of the invention, this consists of a piezoelectric typemotor, known per se, which is capable of converting an input voltagedirectly into a linear displacement of one of its components (automaticdevice); according to an alternative embodiment of the invention, itadvantageously consists, for example, of an eccentric cam, as is wellknown in mechanics (manual device).

Electronic control unit. This typically comprises a microprocessor,which is known per se, and at least one read only memory (ROM) of theknown type, the function of which will be explained in greater detailbelow, and is characterized by the fact that it comprises the amplifiersof the emitter photocurrents i_(A), i_(B), i_(C) and i_(D) forgenerating the voltage signals V_(A), V_(B), V_(C) and V_(D) mentionedabove, operational amplifiers for determining the difference between twosignals and comparators for comparing two signals, all of which arestandardized electronic circuits which are known per se.

The method with which the opto-electronic device 10 is used formeasurement of the vertical alignment of a head according to theinvention will now be described with reference to FIGS. 4a-4 g, underthe (non-limiting) assumption that the vertical misalignment of the headwith respect to the theoretical position is limited to within ±125 μm,and that each of the photosensitive areas A, B, C and D has arectangular shape of dimensions 250×50 μm. FIGS. 4a-4 g show by way ofexample some of the possible relative positions of the circular lightspot 30 and the photosensitive areas A, B, C and D at the instant whenthe head, transported by the scanning carriage, passes exactly oppositethe circular light spot 30 (and hence the centre of the circle 30corresponds with the vertical fine L which separates the areas A and Brespectively from the areas D and C, shown in FIG. 3a).

Taking into account the relationship between the dimensions of thecircular fight spot 30 and that of the photosensitive areas A, B, C andD mentioned above, it can be readily seen that the position of thecircular fight spot 30 with respect to the photosensitive areas A, B, Cand D of the opto-electronic device 10 integrated in the head will varybetween a condition where there is total overlapping and a positionwhere there is no overlapping. FIGS. 4a-4 g show by way of example sevendifferent conditions ({circumflex over (1)}-{circumflex over (7)} lyingbetween these two extremes, the condition {circumflex over (4)} (FIG.4d) being the theoretical one of vertical alignment, in which the centreof the circle 30 coincides exactly with the centre of the rectangleformed by the set of photosensitive areas A, B, C and D according toFIG. 3a.

For detection of the vertical alignment it is sufficient to use a singlepair of phototransistors of the opto-electronic device 10, for examplethe pair T_(A) 30 and T_(B) 40. Indicating with V_(A) and V_(B)respectively the output voltages from the abovementioned amplificationcircuits, connected to the emitters of the phototransistors T_(A) 30 andT_(B) 40 with the corresponding photosensitive areas A and B, FIG. 5ashows in schematic form the pattern of the values of V_(A) and V_(B) ineach of the conditions {circumflex over (1)}-{circumflex over (7)} ofFIGS. 4a-4 g, while FIG. 5b shows in schematic form the pattern of thevalues of the difference (V_(A)−V_(B)) again in each of the conditions({circumflex over (1)}-{circumflex over (7)} of FIGS. 4a-4 g. It can bereadily observed that the values of the difference (V_(A)−V_(B)), at theinstant when the centre of the circular area 30 corresponds with thevertical line L which separates the areas A and B respectively from theareas D and C, represent the vertical misalignment of the head withrespect to the theoretical position, whereby

(V _(A) −V _(B))=0  (1)

is the theoretical condition of perfect alignment.

The method with which the optoelectronic device 10 is used to determinethe condition of horizontal alignment between the centre of the circularlight spot 30 and the vertical line L which separates the areas A, Brespectively from the areas C, D—mentioned above as being a necessarycondition for performing the measurement of the vertical alignment of ahead according to the invention—will now be described.

In order to determine whether this condition has occurred it isnecessary to use all four vertical phototransistors T_(A) 30, T_(B) 40,T_(C) 50 and T_(D) 60 which form the opto-electronic device 10,comparing with one another the pairs of signals V_(A), V_(D) and V_(B),V_(C), it can be immediately understood, also on the basis of FIGS. 4a-4g, that the condition where the centre of the circle 30 coincides withthe vertical line L which separates the areas A and B respectively fromthe areas D and C corresponds to the simultaneous occurrence of the twoconditions

(V _(A) =V _(D)) and (V _(B) =V _(C)),  (2)

excluding the condition

V _(A) =V _(D) =V _(B) =V _(C)=0  (3)

which means that the phototransistors T_(A) 30, T_(B) 40, T_(C) 50 andT_(D) 60 are not illuminated.

The electronic control unit of the printer is able to verify, usingmeans which are known to those skilled in the sector art, the presenceof these conditions and hence perform correctly also the measurement ofthe vertical alignment.

It is now possible to describe the alignment system for multiple colourink jet printheads according to the invention, containing, respectively,for example, a black ink, a blue ink, a yellow ink and a magenta ink andmounted on the scanning carriage of a printer, which is in turn providedwith the illumination device, the device for performing verticaldisplacement of the printheads and the electronic control unit which aredescribed above.

Said alignment system substantially comprises the following steps:

a first step consisting of detection of the horizontal misalignment ofeach head;

a second step consisting of detection of the vertical misalignment ofeach head;

a third step consisting of correction of the timing of the emission ofthe droplets by each individual head, so as to compensate for thehorizontal misalignment detected during the first step;

a fourth step consisting of correction of the vertical position of eachindividual head, detected during the second step.

First step. The detection of the horizontal misalignment of a head withrespect to the theoretical position involves the measurement of thetemporal deviation between the instant to in which the head itself;transported by the scanning carriage, passes exactly opposite thecircular light spot 30 (and hence the centre of the circle 30corresponds with the vertical line L which separates the areas A and Brespectively from the areas D and C, shown in FIG. 3a), and thepredetermined theoretical instant to in which the event should occur.

Said deviation will have a plus sign and value equal to Δt_(A), in thecase where the head is horizontally displaced in the carriage scanningdirection with respect to the theoretical position which it shouldoccupy (namely it arrives in advance); and will have a minus sign andvalue equal to Δt_(R), in the case where the head is horizontallydisplaced in the opposite direction to the carriage scanning directionwith respect to the theoretical position which it should occupy (namelyit arrives with a delay).

This measurement is performed by the electronic control unit of theprinter using methods which are well known to those skilled in thesector art, for example, counting how many pulses emitted by anoscillator of known frequency are contained between the theoreticalinstant t₀, calculated on the basis of the position in which theillumination device is fixed, and the actual instant t₁ in which thesaid condition of horizontal alignment occurs.

Second step. The detection of the vertical alignment of a head withrespect to its theoretical position involves the measurement of thevalue of the difference (V_(A)−V_(B)) in the horizontal alignmentcondition, i.e. in the instant t₁, detected during the first step.

Third step. The correction of the timing of the emission of the dropletsis performed by the electronic control unit of the printer delaying,with respect to the theoretically determined instant t₀, the emission ofthe ink droplets by the heads which are horizontally displaced in theopposite direction to the carriage scanning direction with respect tothe theoretical position which they should occupy, by a time periodequal to Δt_(R); and by advancing, again with respect to thetheoretically determined instant, the emission of the ink droplets bythe heads which are horizontally displaced in the carriage scanningdirection with respect to the theoretical direction which they shouldoccupy, by a time period to Δt_(A).

Fourth step. The correction of the vertical position is performed by theelectronic control unit of the printer for the heads which, on the basisof the detection performed during the second step, are verticallymisaligned, in accordance with a preferred embodiment of the presentinvention, by means of suitable operation of a piezoelectric motor so asto displace them vertically downwards or upwards. Determination of theexact degree of the displacement to be performed is effected by theelectronic control unit of the printer on the basis of a “look-up table”for carrying out conversion between the values of the difference(V_(A)−V_(B)) and the microns of misalignment which they represent,which is stored, for example, in a ROM and predefined on the basis ofthe known characteristics (geometric, optical and electrical) of thecomponents involved in the measurement (optical device, illuminationdevice, amplifiers, etc.).

Obviously it is possible to make modifications and additions to theinvention described above, without thereby departing from the protectivescope thereof

For example, correction of the vertical position, corresponding to thefourth step of the alignment system according to the invention, may beperformed by means of suitable operation by an operator of an eccentriccam so as to displace vertically downwards or upwards the head by acertain amount, via subsequent adjustments determined on the basis of anindication of the result obtained, which is displaced by the electroniccontrol unit of the printer.

Or else it is possible, by way of an alternative to correction of thevertical position and assuming that, even with a vertical displacementof half a pitch, an acceptable print quality is nevertheless obtained,to shift the driving of the nozzles by one or more places along thenozzle array. In this way it is possible, for example, to dispense withuse of the nozzles of each head which are located outside a commonhorizontal band. In particular, in the case of a pitch between thenozzles equal to {fraction (1/600)} of an inch (≈42 μm), still assuminga maximum vertical misalignment between the heads of ±125 μm, in theworst case, the use of six nozzles could be dispensed with.

Moreover it is possible to carry out an additional function, consistingof the possibility of automatically calibrating the luminous intensityof the circular spot 30 according to the sensitivity of thephototransistors so as to obtain values of the emitter photocurrentsi_(A), i_(B), i_(C) and i_(D) which are constant from head to head; thisrequires the integration of a calibration photosensor T_(E) with aphotosensitive area E (see FIG. 3b) having a height equal to at leastthe value of the assumed maximum misalignment, for example 500 μm. Theelectric power supplied to the light source of the illumination deviceis adjusted automatically so as to obtain a predetermined value for themaximum emitter photocurrent i_(E) of the calibration photosensor T_(E),and this adjustment is then maintained for the subsequent head alignmentdetection step described above.

Finally, it is possible to use the optoelectronic device 10 integratedon the chip of the head, by means of obvious modifications of the systemdescribed, for adjustments in alignment during processing steps withinthe process for production of the head itself or the printer, such asfor example:

adjusting both the horizontal and vertical mechanical alignment of thesupport seats of the printheads on the scanning carriage of a colour inkjet printer provided with multiple heads, during testing of the printeritself or during a maintenance operation, using a sample printhead, or:

adjusting both the horizontal and vertical mechanical alignment of thechip+flat cable assembly subgroup with respect to the container of aprinthead, during construction of the head itself, or also

adjusting both the horizontal and vertical mechanical alignment,relative to one another, of the two or more chips which form a multipleprinthead (able to print in a single pass a strip with a heighttypically greater than one inch) for particular applications (forexample, postal franking).

In short, without prejudice to the principle of the present invention,the constructional details and the embodiments may be greatly variedwith respect to that described and illustrated, without departing fromthe scope of the invention itself.

What is claimed is:
 1. Alignment system for multiple colour ink jetprintheads in a dot matrix printer, said printer comprising a fixedstructure; a scanning carriage for supporting said heads, which ismovable with respect to said fixed structure in a first horizontaldirection; and an electronic control unit comprising a function fortiming the emission of ink droplets by said heads, characterized in thatsaid alignment system comprises the steps of: providing printheadscomprising a built-in optoelectronic position detector, for detecting afirst misalignment of said heads in said first horizontal direction anda second misalignment of said heads in a second vertical direction,substantially perpendicular with respect to said first horizontaldirection, said heads also comprising a plurality of nozzles arranged atuniform intervals in at least one row parallel to said second verticaldirection; providing an illumination device integral with said fixedstructure for producing a light spot focused on said opto-electronicdetector; detecting said first misalignment and said second misalignmentby means of the combined use of said built-in optoelectronic positiondetector and said illumination device; compensating for said firstmisalignment of said heads in said first horizontal direction, by meansof a variation of said timing of said emission of ink droplets; andcompensating for said second misalignment of said heads in said secondvertical direction by means of a vertical displacement of said heads. 2.Alignment system according to claim 1, characterized in that saidvertical displacement of said heads is performed by means ofpiezoelectric motors.
 3. Alignment system according to claim 1,characterized in that said vertical displacement of said heads isperformed by means of rotation of eccentric cams.
 4. Alignment systemaccording to claim 1, characterized in that said second misalignment ofsaid heads in said second vertical direction is corrected by shiftingthe driving of the nozzles by one or more places along the nozzle array.5. Alignment system according to claim 1, characterized in that saidbuilt-in opto-electronic position detector is of the four-quadrant type.6. Alignment system according to claim 5, characterized in that saidbuilt-in opto-electronic position detector comprises four verticalphototransistors with an open base and common collector, each having anindependent emitter.
 7. Alignment system according to claim 6,characterized in that said built-in opto-electronic position detectoralso comprises a vertical calibration phototransistor.
 8. Alignmentsystem according to claim 6, characterized in that said four verticalphototransistors are substantially identical to one another. 9.Alignment system according to claim 8, characterized in that said openbase has a photosensitive surface in the form of a rectangle with alarger side parallel to said second vertical direction and with asmaller side parallel to said first horizontal direction.
 10. Alignmentsystem according to claim 9, characterized in that said larger side hasdimensions of between 100 and 500 μm and said smaller side hasdimensions of between 10 and 200 μm.
 11. Alignment system according toclaim 8, characterized in that said first misalignment of said heads insaid first horizontal direction is detected by means of checking for acondition of equality of the photocurrents from said independentemitters of said four vertical phototransistors.
 12. Alignment systemaccording to claim 8, characterized in that said misalignment of saidheads in said vertical direction is detected by means of checking for acondition of equality of the photocurrents from said independentemitters of only two of said four vertical phototransistors. 13.Alignment system according to claim 1, characterized in that saidillumination device comprises a light generating device chosen from thegroup composed of laser diodes and light emitting diodes (LED's). 14.Alignment system according to claim 1, characterized in that saidillumination device comprises a lens system for focusing said light spoton said built-in opto-electronic position detector.
 15. Alignment systemaccording to claim 14, characterized in that said illumination devicealso comprises a circular diaphragm.
 16. Alignment system according toclaim 1, characterized in that said multiple printheads are four innumber and contain respectively a black ink, a blue ink, a yellow inkand a magenta ink.
 17. Alignment system for multiple colour ink jetprintheads in a dot matrix printer, said printer comprising a fixedstructure, a scanning carriage for supporting said heads, which ismovable with respect to said fixed structure in a first horizontaldirection; and an electronic control unit comprising a function fortiming the emission of ink droplets by said heads, characterized in thatsaid alignment system comprises the steps of: providing printheadscomprising a built-in opto-electronic position detector, for detecting afirst misalignment of said heads in said first horizontal direction anda second misalignment of said heads in a second vertical direction,substantially perpendicular with respect to said first horizontaldirection, said heads also comprising a plurality of nozzles arranged atuniform intervals in at least one row parallel to said second verticaldirection; providing an illumination device integral with said fixedstructure for producing a light spot focused on said optoelectronicdetector, detecting said first misalignment and said second misalignmentby means of the combined use of said built-in opto-electronic positiondetector and said illumination device; compensating for said firstmisalignment of said heads in said first horizontal direction, by meansof a variation of said timing of said emission of ink droplets; andcompensating for said second misalignment of said heads in said secondvertical direction by means of the selective non-use of some of saidplurality of nozzles.
 18. Alignment system according to claim 17,characterized in that said built-in opto-electronic position detector isof the four-quadrant type.
 19. Alignment system according to claim 18,characterized in that said built-in opto-electronic position detectorcomprises four vertical transistors with an open base and commoncollector, each having an independent emitter.
 20. Bubble ink jetprinthead comprising: a semiconductor substrate; a first plurality ofemission elements integrated on said substrate, for producing inkdroplets through a corresponding plurality of nozzles which are arrangedat uniform intervals in at least one row in a first vertical direction;a second plurality of electronic components integrated on said substrateby means of MOS technology for selectively and driving said firstplurality of emission elements, characterized in that it also comprisesan opto-electronic position detector of the four-quadrant type,integrated on said substrate by means of said MOS technology, saidopto-electronic position detector being provided for cooperating with anexternal illumination device.
 21. Printhead according to claim 20,characterized in that said substrate is silicon and said MOS technologyis chosen from a group consisting of CMOS technology and LDMOStechnology.
 22. Printhead according to claim 20, characterized in thatsaid built-in opto-electronic position detector comprises four verticalphototransistors with an open base and common collector, each having anindependent emitter.
 23. Printhead according to claim 22, characterizedin that said built-in opto-electronic position detector also comprises avertical calibration phototransistor.
 24. Printhead according to claim22, characterized in that said four vertical phototransistors aresubstantially identical to each other.
 25. Printhead according to claim24, characterized in that said open base has a photosensitive area inthe form of a rectangle with a larger side parallel to said firstvertical direction and with a smaller side parallel to a secondhorizontal direction perpendicular to said first vertical direction. 26.Printhead according to claim 25, characterized in that said larger sidehas dimensions of between 100 and 500 μm and in that said smaller sidehas dimensions of between 10 and 200 μm.
 27. Bubble ink jet printercomprising a fixed structure; a scanning carriage for supporting aplurality of printheads, which is movable with respect to said fixedstructure in a first horizontal direction; and an electronic controlunit having a function for timing said emission of ink droplets by saidheads, characterized in that said printheads comprise a built-inopto-electronic position detector for detecting a first misalignment ofsaid heads in said first horizontal direction and a second misalignmentof said heads in a second vertical direction, substantiallyperpendicular with respect to said first horizontal direction; and inthat said printer also comprises an illumination device integral withsaid fixed structure for producing a light spot focused on said built-inopto-electronic position detector.
 28. Printer according to claim 27,characterized in that said timing is conditioned by said firstmisalignment detected by means of said built-in opto-electronic positiondetector.
 29. Printer according to claim 27, characterized in that italso comprises means for performing vertical displacement of said headsand in that said displacement is conditioned by said second misalignmentdetected by means of said built-in opto-electronic position detector.30. Printer according to claim 27, characterized in that said built-inopto-electronic position detector is of the four-quadrant type. 31.Printer according to claim 30, characterized in that said built-inoptoelectronic position detector comprises four verticalphototransistors with an open base and common collector, each having anindependent emitter.
 32. Printer according to claim 29, characterized inthat said means for performing said vertical displacement of said headscomprise a piezoelectric motor.
 33. Printer according to claim 29,characterized in that said means for performing said verticaldisplacement of said heads comprise eccentric cams.
 34. Printeraccording to claim 27, characterized in that said illumination devicecomprises a light generating device chosen from the group composed oflaser diodes and light emitting diodes (LED's).
 35. Printer according toclaim 27, characterized in that said illumination device comprises alens system for focusing said light spot on said built-inopto-electronic position detector.